Method of making fluorocarbons



Patented Dec. 14, 1948 ma'rnon or MAKING 'rwoaocannons Joseph H. Simona,State College, Pa., assignor to MinnesotaMining 8: ManufacturingCompany, St. Paul, Minn., a corporation of Delaware No Drawing.Application April 1, 1944,

Serial No. 529,182

4 Claims.

This invention relates to new chemical compounds of carbon and fluorineand to a novel and simple rocess for the preparation of these newcompounds.

This application is a continuation-in-part of my copending applicationSerial No. 237,157 filed October 26, 1938, now abandoned...

An object of this invention is to provide a new and useful catalyticprocess'for the preparation of fluorocarbons by the reaction of carbonand fluorine. A feature of this process is that it permits of obtainingsubstantial yields of fluorocarbons containing three'or more carbonatoms in the molecule, at reaction temperatures in the range of 200 to600 C., and the reaction is smooth and non-explosive. This novel processmakes it possible not only to prepare fluorocarbons of the aliphatictype (CnFimi-Z), but also cyclic types.

This process makes possible the production of normally liquid aliphaticfluorocarbons (having boiling points above C.) These contain five ormore carbonatoms in the molecule, are chemically inert, and arethermally stable at temperatures up to at least 750 C'.

It has been known that lower molecular weight organic compoundscontaining fluorine could be prepared by a number of different methods.Primarily such compounds have consisted of partially or completelysubstituted fluorine derivatives of methane, such as mono-, di-, tri-,and tetra-fluoro methane, CHzF, CH2F2, CHFa, and cm. These compoundshave in general been prepared by the'reaction of fluorinating agentssuch as antimony fluoride or silver fluoride with halogenated aliphatichydrocarbons, or in particular by special methods'for each compound.

It has also been known that carbon and fluorine will react at relativelylow temperatures ranging from approximate1y280 C. to 420 C.,

depending upon the form of carbon used, to produce carbon monofluoride(CF) in the form of a whitish solid composition which is unstable andexplodes violently upon further heating.

. On the other hand, the compounds of carbon and fluorine of thisinvention are characterized by thermal stability at temperatures atleast as high as 1,000-C.

Prior to this invention attempts to produce fluorocarbons by directlyreacting carbon with fluorine have resulted in explosions unless carriedout at temperatures not substantially less than 700 C. and at these hightemperatures it has been found that the only compounds of carbon andfluorine which could be produced in identifiable quantities were CF; andCzFc.

In order to produce the higher compounds having at least three carbonsto the molecule. it is necessary to conduct the reaction at temperaturesin the range of 200 C. to 600 C. and, as above noted. prior attempts toaccomplish such reactions at these temperatures have resulted only inviolent explosions.

I have discovered that by conducting the reaction of carbon and fluorinein the presence of fluorides ,of mercury, antimony, aluminum and iron, acalm reaction is obtained at temperatures in the range of 200 C. to 600C. and that the product of such reaction is a mixture of fluorocarbonsincluding substantial proportions of fluorocarbons having three or morecarbon atoms to the molecule. In general, the proportion of thecompounds having three or more carbon atoms per molecule is increased asthe temperature at which a calm reaction is obtainable is decreased. Thefluorides above mentioned act as catalysts for the reaction and may beintroduced as such or produced in situ from the respective elements orother compounds there f since by reason of the extreme chemical acti tyof fluorine, either the elements or their compounds are readilyconverted into the corresponding fluorides at the temperatures used inthese reactions.

Furthermore, I have succeeded in isolating and characterizing certain ofthe higher fluorocarbons resulting from the above mentioned process, aswell as certain mixtures of higher fluorocarbons as hereinafter setforth. To the best of my knowledge, neither these fluorocarbons northese mixtures have been produced prior to my discovery and thus theyconstitute new compositions of matter produced for the first time by thepractice of my process.

These compounds may be generally characterized as thermally stable,non-oleflnic fluorocarbons. By analogy to well-known terminologyapplicable to hydrocarbons, they may be further characterized as: (A),compounds of the aliphatic series, (B), compounds of the alicyclicseries, (C),

compounds of the aromatic 0r benzenoid series. With respect to thealiphatic series, mentioned above, the only compounds heretoforedefinitely identified are CF4 and CzFe. of Cal; and C4F10 have beenpostulated, yet they have never heretofore been isolated. orcharacterized. No compounds of either the alicyclic or aromaticserieshave been heretofore obtained.

The physical properties of the new fluorocarbons such as their meltingpoint, boiling point, molecular weight, liquid density, heat of vapori-While the existence anew The above fluorocarbons and also the mixtureshereafter described are all colorless when liquid 'or gaseous, and whiteor colorless when solid. They are odorless, non-toxic, non-corrosive andof little chemical activity. They do not react with strong acid or withstrong alkali at ordinary temperatures. They do not hydrolize withwater. They do not burn or react with oxygen. They do not react with anyof the metals at ordinary temperatures and react only with the morechemically active metals, such as sodium, at elevated temperatures(about red heat). The term "chemically inert as used in the appendedclaims refers to and is limited by the above noted characteristics. Theydo not react with Pyrex" laboratory type glass up to the fusiontemperature of that material, viz. 750 C. Furthermore, at suchtemperatures there is no decomposition of these fluorocarbons. The termthermally stable as used in the appended claims refers to the abovecharacteristics at temperatures up to 750 C. When in a liquid state theabove fluorocarbcns show a pronounced solvent action on greases and wax.They are useful as solvents, refrigerants, starting compounds forsynthesis and a number of other uses.

It has been found that these new fluorocarbons may be obtained byfractional distillation of the crude reaction product obtained byheating finely divided carbon in an atmosphere of fluorine whileemploying a catalyst of the character above mentioned. Previous attemptsto prepare fluorocarbons by reacting carbon with fluorine have not onlyfailed by reason of the resulting explosions production of carbontetrafiuoride (CF!) and hexafluoroethane (CfaFe) which togetheraccounted for approximately 95% of the product of these reactions, theremainder being unidentified .material boiling above -78 C. This smallresidbut have also been substantially limited to the zation, and vaporpressure are given in the 101- composed of 10% ormore o! fluorooarbonshaving lowing table. three-or more carbon atoms per molecule. is a TsarI 7 Pulled propmlu c/flwcwtm 11.1". up. M 1. v .Prees. me. n... a. wi.-;-,g- 35; 00::Pmm.)

n. -m.... -sa ms r45 um +1.o

04min... 1a.... -u no 1.5: 5,000 +1.a

0mm to an M00 '.'-+1.44 cm. ca. -12. a m 1.12 can -%+1.4a

OeFu Bi zoo-s 1.66 6.900 -%+1.8l

my on so 360-6 1.83 1,400 +1 .4a

'r in degrees absolute.)

7 great improvement over anything in any way suggested in the prior art;and that such a reaction product containing fifteen, twenty ortwenty-five 30 percent of such'hish'er fluorocarbons is still more noveland advantageous. By the use of metallic catalysts I have been able notonly to conduct a smooth reaction free from explosions but have alsobeen able to obtain substantial quantities of a5 a reaction product ofwhich from V; to were the compounds of fluorine and carbon having threeor more carbone atoms to the molecule, thus permitting for the firsttime the isolation and determination of properties of these higher 40fluorooarbons'.

One process which may be employed for preparing these new fluorocarbonsis the following:

One of the elements, or a compound thereof, eiiective in catalyzing thereaction is intimately mixed with flnely divided carbon in a quantityequivalent to from 1 to 5%, depending upon the type of catalystemployed. The carbon and catalyst mixture is then placed in a reactionchamber which is heated to the temperature necessary to effect thereaction while gaseous fluorine is passed through the chamber. I haveobtained satisfactory results at temperatures 01' from approximately 200to 600 C. but temperatures of the order of 300 C. are preferred, notonly as favoring the production of the desired fluorocarbons of highermolecular weight, but also as aflording an adequate margin of safetyagainst explosion.

It should be understood that both the percentage of catalyst employedand the temperature at which the reaction 'is conducted vary dependingupon the kind of carbon used. Thus graphite requires a temperaturein thehigher portion of the above range, whereas an adsorption gre do ofcharcoal such "asthat known by the trade name of Norit has been found togive excellent results at the lower temperatures. From the .outlet ofthe reaction chamber the gaseous and vaporous products are passedthrougha tank of an alkaline solution to remove such portions thereof as mayconstitute 'unreacted fluorine, hydrogen fluoride, or other impurities,afterwhich they are led into a gasometer, wherethey are collected overwater. To effect the separation of the various fluorocarbons thusproduced, the gaseous products are action product of fluorine andcarbon, which is condensed from thustorage chamber by passing scribedwill be obvious to aasaoav them through a freezing chamber in which thecooling medium is preferably liquid air. After first removing CF; at itsboiling point of approximately -128 C. and, CaFs at approximately -'I8C.; octafiuoropropane (CaFa) is obtained at approximately 38 C.; the twoisomeric fractions of decafiuorobutane C4F1o #1 and C4F1o #2 areobtained at temperatures of approximately -,-4.'7 C. and +3 0.,respectively; the five-carbon atom fluorocarbon (CsFm) is obtained atapproximately 23 (3.; while the six-carbon atom fluorocarbon (CcFn) andthe seven-carbon atom fluorocarbon (C-1F14) are obtained at temperaturesof approximately 51 and 80 C. respectively.

To purify the new compounds thus obtained each fraction isrefractionated in the same manner and then passed through chamberscontaining potassium hyd oxide and phosphorous pentoxide. Any airremaining in the various fiuorocarbons may be removed by alternatevaporization, condensation and pumping.

In addition to the above definite boiling compounds, fractions wereremoved over the temperature range of from 23 C. to 51 C.; from 51 C. to

- 80 C.; and from 80 C. to 95 0., after which there remained a residuewhich was subsequently determined to consist of fluorocarbons boiling inexcess of 95 C. In each of the above identified fractions the materialwas found to have a progressively increasing boiling point within thestated limits showing that the same consisted of mix-- tures offluorocarbons not separable under fractionating conditions used toseparate the definite boiling point compounds.

The process for producing these new com- I pounds is not confined to thematerials and steps herein specifically recited. As above indicated,different kinds of powdered or granulated carbon may be employed. Thecatalyst employed may be obtained from the actionof fluorine on mercury,antimony, aluminum or iron, or compounds containing any one thereof, orfrom a combination of two or more of the above metals or compoundsthereof. The reaction has been satisfactorily effected without the useof a separate catalyst as such by using a reaction chamber made ofmercury-amalgamated copper tubing or containing a mercury-amalgamatedcopper screening, thereby providing mercury for catalytic action. Thetemperature at which the reaction is carried out is not critical proding it be sufficiently high to effect the combination ofthe carbon andfluorine.

in the process above dethose skilled in the art. Exemplary of oneprocess which has been employed toproduce the new fluorocarbons is the.following procedure: v

Finely divided Norit" was placed in a copper reaction tube provided atone end with an inlet and at the other end with an outlet. The coppertube .was approximately 25 inches long and 2 inches in diameter. TheNorit contained crystals of mercuric chloride in the ratio of 1 gram toOther possible variations 50 ofthe carbon. A roll of amalgamated copperscreening was placed in each end of the reaction chamber. Fluorine gaswas passed through the tube and the tube was heated to a temperaturebetween 400 and 600 C. From the outlet of the reaction chamber, theproducts in gaseous and vapor form were led through a sodium hydroxidedensation in a liquid air trap and subsequent TABLE II 13.1. Deg. VolumePer cent by (5. (liquid) Volume Parts 128 246 54. 5 78. 2 57 12. 6 -3837 8.2 4. 7 24 5. 3 3. 0 6 1.3 23 25 5. 5 23 to 61 14 3. 1 61 Z) 4. 4 5!to 7 1. 5 80 6 l. 3 80 to 4 9 Residue 95 to 7 l. 5

Total 435 100. 0

A further exemplary process is the following:

Finely divided Nuchar" was mixed with approximately 2 percent by weightof mercuric oxide and placed in a boat made of sheet iron. This boat wasthen placed in a 2 inch iron pipe approximately 2 feet long. The ironpipe was placed in an electric furnace and maintained at a temperaturebetween 250 to 300 C. Fluorine gas from the generator and withoutpurification was passed through this iron tube at a rate ofapproximately 3 liters per hour. The products from the reaction tubewere condensed in a'chamber kept at the temperature of liquid air andwere subsequently purified and fractionated. The purification consistedof a treatment with the concentrated sodium hydroxide solution and atreatment with phosphorous pent'oxide. The products formed hadproperties and characteristics of approximately that given in theprevious example. More than one-half of the material remained in theliquid statewhen at temperature '--78 C., from which it is apparent thata higher ratio of fluorocarbons having three or more carbon atoms to themolecule was obtained than in the preceding example.

The above procedure was repeated except that instead of mercuric oxide,powdered elemental antimony was used in an amount of 4% of the weight ofthe carbon. Approximately of the total of the reaction product was foundto remain in theliquid state at '78 C. and to consist of fluorocarbonshaving more than three carbon 'atoms to the molecule.

Other catalysts which promote a smooth, nonsolution and into a gasometerwhere they were.

collected over water. Under the above gconditions, it was liters ofreaction products per hour.

found possible to'cc11ect abciit -1% orine are iron and aluminum. I havealso used successfully, mixtures of mercury with the other abovementioned catalysts. In all cases in which these catalysts or mixturesof catalysts are used,

jthe temperatures at which a smooth, nonexplosive reaction is obtainedare below 600 C.,

M aesaoav but I have found that mercury and its compounds permitted thegreatest reduction in temperatures and thereby induced the greatestshift toward higher molecular weight products. Additionally, I havefound that no significant portion of mercury fluoride or aluminumfluoride passes over with the reaction product, whereas when antimony isused the volatility of the fluoride is such that it passes ofi rapidlywith the reaction product and must be replenished.

The following is further detailed information with respectto thechemical and physical characteristics and also the utility of thevarious fluorocarbons and mixtures thereof which I have obtained by mynew process above disclosed.

OCTAFLUOROPROPANE One of the new compounds is the open chain oraliphatic fluorocarbon containing three carbon atoms per molecule andcorresponding to the formula CaFa. This new compound possessesproperties very much in agreement with those postulated from theoreticalconsiderations. Its molecular weight as determined by means of thedensity balance was found to be 188 in direct conformance with thetheoretical weight for 031%. Other physical characteristics are given inTable I. Separate analyses of this material for the respective elementsgive results of fluorine 80.5% and carbon 19.9%, which establishes afluorine to carbon atomic ratio of 2.56. The corresponding theoreticaldata would befluorine 80.85%, carbon 19.15%, atomic ratio 2.67.

This inert material with its very long liquid range at low temperaturesis an ideal refrigerant. It is also useful as a low temperature heattransfer liquid and a low temperature solvent.

DECAFLUOROBUTANE Other members of this new series of compounds are thetwo isomeric forms of the open chain or aliphatic fluorocarboncontaining four carbon atoms to the molecule and corresponding to theformula C4F1o. Two distinct fractions were obtained having boilingpoints of --4.7 C. and 3.0 C. respectively. This together with otherdetermined characteristics indicates that these decafluorobutanes orchain compounds are analogous to normal butane and isobutane. Themolecular weights of the two fractions were'determined as 230 and 241respectively, deviating somewhat from the theoretical 238 due to thedifficulty in separating two substances with such a small difference inboiling temperatures. The melting points were difiicult to determine,since. both fractions formed a glass when condensed from the vapor.Other physical properties of these two compounds also differ as shown inTable I.

Separate analyses of the lower boiling isomer showed fluorine 80.5%,carbon 18.8%, with fluorine to carbon atomic ratio of 2.7(theoreticalfluorine 79.85%, carbon 20.15%, atomic ratio of 2.5).Analysis of the higher boiling isomer showed fluorine 78.8% andafluorine to carbon molecular atomic ratio of 2.34. The average fluorineto CsFm Another compound which I have obtained is the ring or alicyclicfluorocarbon containing flve carbon atoms per molecule and correspondingto the formula Csl 'io. Separate analyses of this material showedfluorine 76.4 carbon 24.2% giving OF; 0F! CF:

Fa-OFI I since its molecular weight has beendetermined to be between 254and 259, conforming with the theoretical molecular weight of 250 forC'sFio.

The presence of some chain compounds of the formula CaFn boiling at thesame temperature range probably accounts for the discrepancy inmolecular weight of this faction over the theoretical molecular weight.Other physical properties of this compound are given above in Table I.

This compound is particularly useful for a low boiling point solvent, afire extinguisher or a heat transfer medium. I

Another of these compounds is the ring or alicyclic fluorocarbon,boiling at 51 C. and having six carbon atoms per molecule. It was foundto have a molecular weight between 300 and 303, thus corresponding withthe theoretical molecular weight of 300 for CcFrz. This compound tendsto form crystals when condensed from the vapor, in small amounts, butlarger amounts form a glass. A melting point for this compound was notobtained. Separate analysis showed fluorine 76.1%, carbon 24.6%, givinga fluorine to carbon atomic ratio of 1.955 (theoretical-fluorine 76%,carbon 24%, atomic ratio 2.00). Other physical properties of thismaterial will be found in Table I, is given above.

This material is an excellent solvent, 8. fire extinguisher, anextraction medium, an inert medium for chemical reactions, and a'heattransfer medium.

A compound corresponding to the formula C'zFu is another of the newlydiscovered ring or alicyclic fluorocarbons. This compound boils at 80 C.and has a molecular weight between 350 and 355. The theoreticalmolecular weight for a 01F com pound is 350. Like the six cat-lion atomfluorocarbon, this compound tends to form crystals when condensed insmall amounts from its vapor but larger amounts form a glass.Time-temperature curves for both the six-carbon-atom fluorocarbon andthe seven-carbon-atom fluorocarbon show no melting points. Analysisshowed fluorine 76%, carbon 23.1 giving a fluorine to carbon atomicratio of 2.08, (theoretical-fluorine 76%, carbon 24%, atomic ratio2.00). Other physical pgoperties of this compound are given in Table I aove.

This material is particularly useful as a solvent, as an inert mediumfor chemical reactions, as a hydraulic liquid, as a heat transfer mediumand as a dielectric.

In addition to the compounds having definite boiling points asenumerated above and also as tabulated in Table I, which have beenisolated and identified as the definite fluorocarbons above increasingboiling point within the limits set forth in Table II.

Fluorocarbon mixture boiling between 23 C. and

, This fraction, which showed a continuously increasing boiling point,was removed subsequently to the constant boiling compound CsFio andprior to the constant boiling compound CsFia. The average molecularweight of this fraction was found to be 302. It was found by separateanalyses to contain 79.2% fluorine and 22% carbon which gives a fluorineto carbon atomic ratio of 2.3. As the fluorine to carbon atomic ratio ofCsFm, removed immediately prior and of CsFn,

removed immediately subsequent to the fraction is, in each case, 2.00,this definitely proves that this fraction boiling between 23 C. to 51.C. must contain principally chain or aliphatic compounds. The chain oraliphatic compounds which would be expected to boil in the range wouldbe the isomers of the five and six carbon-chain type fluorocarbons.These have fluorine to carbon atomic ratios of 2.4 and 2.33 andmolecular weights of 288 and 338,respectively. The fact that theexperimentally determined fluorine to carbon atomic ratio for themixture boiling at 23 C. to 51 C. was 2.3, whereas the theoreticalratios for CsFi: and CsFu are. respectively, 2.4 and 2.33, indicatescontamination with the flve and six carbon alicyclic compounds with aratio of 2.00 or with a very small proportion of'a benzenoidfluorocarbon.

This material is useful as a solvent, an extraction liquid, a fireextinguishing liquid. an inert medium for chemical reactions. and as aheat transfer medium.

Fluorocarbon mirture boiling between 80 C. and

This fraction also distilled at continuously increasing temperature andwas removed immediately after the 01F fraction boiling at 80 C. andincluding material boiling up to 95 C. The average molecular weight inthe fraetion'was l fore, that the fraction of reaction product boilingin the range 8045? C. must contain a substanfound to be 358. The valuesobtained upon analysis were: fluorine 'from'71.3% to 71.8%; carbon.28.4%, giving a fluorine to carbon ratio'of 1.59.

When the vapor of this fraction. together with fl orine, was passed overHgF at 100 C. no'signiflcant change in fluorine analysis was observed.

have been fluorinated. Therefore, the very low fluorine to carbon'atomicratio (1.59) must be tial proportion of aromatic or benzenoidfluorocarbons.

This fraction is most useful as asolvent. as a heat transfer medium, asan inert medium for chemical reactions, as a hydraulic liquid, as a highdensity-extraction liquid and as a dielectric.

fluorocarbon mixture boiling between 51 C. and

This fraction also distilled with continuously increasing temperatureand was removed after the CsFn fraction boiling at 51 C. and immediatelyprior to the C'rFm fraction, boiling at 80 C. It was an inert, thermallystable fluorocarbon mixture. The composition of this fraction isapparent from the compositions .of mixture A, known to contain aliphaticor chain type compounds and mixture C, known to contain aromatic orbenzenoid type compounds. Because of incomplete separation of thealicyclic compounds CsFu-and 01F some of these are also present. Thismixture,B therefore contains representative members of the three seriesof non-oleflnic thermally stable fluorocarbons, namely the aliphatic.the

' aromatic, and the alicyclic.

fluorocarbon mixture boiling above 95 C.

This fraction was found to contain 77% flourine and (by difference) 23%carbon, corresponding to a fluorine to carbon atomic ratio of 2.1 On'account of the large range in boiling point, it was diflicult to obtaina reliable value for average molecular weight; a value of 424 wasobtained experimentally, but is probably to low. However this molecularweight definitely indicates fluorocarbons having eight or more carbonatoms to the molecule. I

For reasons unknown, there appear to be inthough in the next lowerboiling mixture (80 to 95 'C.) conclusive evidence was obtained for avery substantial proportion of such fluorocarbons.

The fact that the experimentally -determined atomic ratio is above 2.00,indicates the presence 1 of chain or aliphatic compounds.

taken as proof that fluorocarbons of the aromatic or benzenoid serieswere. present in substantial proportion. The fluorine to carbon ratiosfor the six-, sevenand eight-carbon fluorocarbons in this serieswouldbe. respectively, 1.00, 1.14 and 1.25. whereas the'ratio for thechain or aliphatic compounds must always be greater than 2 and for thealicyclic compounds boiling in this range. 2.00. (The number of carbonatoms per molecule in compounds boiling within this range could not. belarge enough to permit polycyclic, saturated compounds, with a fluorineto carbon atomic ratio less than 2.00, to be present.) It is evident,there? This material, in addition to being useful as in inert solventorliquid medium is useful as a hydraulicdiquid. a heat transfer medium,a turbine impellent, a transformer liquid, a dielectric and a lubricant.It is also useful as an additive to other materials to confer upon themixtures some of the desirable properties of the fluorocarbons. I

' In the appended claims the term "fluorocarbons" defines compoundsconsisting only of carbon and fluorine. Also the. term- "mercury catalyscomprises mercury. mercuric and mercurous' salts, and mercury containingmetals such as amalgamated metals and the like.

It is tovbe expressly understood that the foregoing description andexamples are illustrative only and are not to be conside'redas limiting"the invention beyond the scope of the claims.

Attentionis called to my con ending-subsequent v 11 applications filedas divisions or continuatlonsin-part of the present application anddirected to certain novel fluorocarbon compounds produced by the methodherein described and claimed, namely, Ser. No. 684,785 (filed July 19,1946), Serial No.v 684,786 (flied July 19, 1946) now abandoned. Ser. No.788,651 (filed Nov. 28, 1947), Ser. No. 789,553 (filed Dec. 3, 1947),and Ser. No. 790,437 (filed Dec. 8, 1947).

I claim:

1. A process for the production of fluorocar buns which comprisesheating carbon to a temperature of from 200 C. to 600 C. in anatmosphere of fluorine and in the presence of at least one material fromthe group consisting of the fluorides of mercury, antimony, aluminum andiron, serving as a catalyst to cause smooth non-explosive reactionbetween the carbon and the fluorine within said temperature range.

2. A process for the production of a fluorocarbon which comprisesheating carbon to an elevated temperature in an atmosphere of fluorineand in the presence of a mercury fluoride catalyst.

3. A process for the produtcion of fluorocarbons which comprisescontacting gaseous fluorine with carbon at a temperature above 400 C. inthe presence of a mercury fluoride catalyst.

4. A process for the production and separation of a fluorocarboncomprising contacting fluorine with carbon at elevated temperatures inthe presence of a mercury fluoride catalyst, and fractionally distillingthe product obtained thereby.

JOSEPH H. SIMONS.

REFERENCES CITED The following references are of record in the file ofthis patent:

Rut! and Keim, Zeit. Anorg. Allgem. Chem, vol. 192, pages 250-1 (1930).

Rufl and Bretschneider, Ibid., vol. 217, pages 2, 7, 10, 19-21 (1934).

Lebeau et a1., "Compt. Rend., vol. 191, pages 939-40 (1930). i

Simons et al., fJour. Am. Chem. Soc. vol. 59, page 1407 (1937).

